In general, vehicle parts, such as automotive parts, are subjected to a coating process after welding to improve the corrosion resistance and appearance. In the process, if there is slag on the weld bead, the coating material may be repelled by the slag, which can result in a coating defect. Thus, for welding of a thin steel sheet for, for example, automotive parts, welding wires that do not generate much slag, such as a solid wire and a metallic flux-cored wire, are conventionally used.
Welding is a process that achieves joining by melting metal, and thus an oxide film inevitably forms on the weld, which is, in a heated condition, exposed to atmospheric air. Oxide films hinder good chemical conversion and coating, which can result in a decrease in coating adhesion. In addition, if the adhesion of the oxide film that forms on the weld bead is not sufficient, the oxide film can easily peel off when the coated portion is subjected to an impact, for example, by a bouncing stone, and the oxide film can delaminate together with the coating provided thereon.
Thus, because of the slag and the oxide film that form on the weld bead, it is difficult for the weld to exhibit coating properties comparable to those of the steel sheet. Delamination of a coating results in a decrease in corrosion resistance and corrosion life, and therefore the weld is regarded as an area where corrosion resistance is particularly low. In view of this, various studies have been conducted in the related art in order to improve the coating properties and other properties of the weld (see PTLs 1 to 3, for example).
For example, PTL 1 describes a flux-cored wire having a wire composition in which one or more of C, Si, Mn, Nb, V and P are contained in a specified amount, the content of a slagging agent is less than or equal to 1.0 mass %, and the ratio between FeO and SiO2 (FeO/SiO2) in the slagging agent is greater than or equal to 1.0. This is intended to improve adhesion between the slag and the coating and between the slag and the weld bead.
PTL 2 describes a flux-cored wire in which graphite is added to the flux to reduce the amount of slag formed. Graphite reacts with oxygen in the weld metal to form CO and CO2 gas. Thus, the weld metal is deoxidized, and consequently deoxidation by, for example, Si or Mn is inhibited, and as a result, the amount of slag formed is reduced.
PTL 3 describes a welding wire in which Ni and Cr are added to the welding wire to form a thin adhesive oxide film on the surface of the weld bead. This is intended to improve the delamination resistance of the oxide film and the coating.